Electric cables

ABSTRACT

High voltage electric cables of the kind having a laminated dielectric comprising lappings of insulating tape impregnated with an insulating liquid enclosed in a liquid-impervious sheath or pipe. Such laminated dielectric is most commonly built up from lappings of paper tape although proposals have been made to use plastics film along or interleaved with paper and it has been proposed that plastics film having on one or both sides an adherent layer of fibrous material should be used. More particularly the invention relates to such a cable in which the insulating liquid is a liquefied gas which, in addition to forming part of the dielectric, acts as a cooling medium thus increasing the cable rating for a given conductor cross-section.

[ Nov. 20, 1973 I 1 ELECTRIC CABLES [75] Inventor: Edward Henry Reynolds, London,

England [73] Assignee: British Insulated Callender & Cables Limited, London, England 22 Filed: Feb. 1,1972

21 Appl. No.: 222,582

[52] U.S. Cl 174/25 G, 174/15 C, 174/36,

174/102 SC, 174/110 F, 174/120 FP [51] Int. Cl. H0lb 9/06 [58] Field of Search 174/15 C, 16 B, 256,

174/25 R, 27, 120 R, 120 FP, 120 sc, 102 sc, 105 sc, 106 sc, 110 PM, 107, 110 F,

1,185,474 3/1970 Great Britain 174/120 FP 940,269 10/1963 Great Britain 174/110 PM 874,981 8/1961 Great Britain 174/120 FP 1,108,762 4/1968 Great Britain 174/25.6

OTHER PUBLICATIONS Mulcany et al., A Review of Insulation Breakdown and Switching in Gas lnsulation, lnsulation/Circuits 870, pp. 5561.

Primary Examiner-Bemard A. Gilheany Assistant Examiner-A. T. Grimley Attorney-Eugene F. Buell [57] ABSTRACT High voltage electric cables of the kind having a laminated dielectric comprising lappings of insulating tape impregnated with an insulating liquid enclosed in a liquid-impervious sheath or pipe. Such laminated dielectric is most commonly built up from lappings of paper tape although proposals have been made to use plastics film along or interleaved with paper and it has been proposed that plastics film having on one or both sides an adherent layer of fibrous material should be used. More particularly the invention relates to such a cable in which the insulating liquid is a liquefied gas which, in addition to forming part of the dielectric, acts as a cooling medium thus increasing the cable rating for a given conductor cross-section.

19 Claims, 2 Drawing Figures ELECTRIC CABLES In accordance with the present invention we provide a high voltage cable having a laminated dielectric built up at least in part from tapes comprising an imperforate plastics filmhaving bonded to each of its major surfaces a cellulosic paper tape, preferably of low density and high impermeability, impregnated with a liquefied gas which is maintained at a pressure substantially greater than that of the atmosphere.

Preferred liquefied gases include liquid nitrogen, liquid hydrogen, liquid helium, liquid natural gas, liquid sulphurhexafluoride, and Arctons, e.g. trifluoromethane (boiling point 82C), trifluorochloromethane (boiling point 8 1C) and difluorochloromethane (boiling point 41C).

The liquefied gas impregnating the laminated dielectric and cooling the cable is preferably maintained at a pressure of at least 2 atmospheres, because it has been found that, as compared with a conventional oil impregnated paper dielectric or oil impregnated plastics/- paper laminated dielectric, the a.c. breakdown strength of the laminated dielectric of the present invention is then significantly increased.

The structure of the cable of the present invention may be similar to that of an oil-impregnated cable, the only major additional structural feature of the cable or cable installation that may be required being extra thermal insulation necessary to accommodate for the low operating temperatures, which may be as low as 50C or even considerably lower and which will depend upon the load carrying requirements of the installation of which the cable forms a part and upon the environment and profile of the route in wich the cable is laid. Such additional thermal insulation may comprise a component part of the cable that may lie inside or outside the cable sheath, or it may comprise a material in which the cable is embedded or by which it is surrounded when the cable forms part of a cable installation.

Although the conductor or conductors of the cable may be of conventional metals such as copper or aluminium of high purity, the use of a liquefied gas as the impregnant/refrigerant provides for the use of metals normally unsuitable for the conductors of high voltage cables, e.g. beryllium and magnesium.

The paper tapes and the plastics film are preferably bonded together in such a way as to produce a prestressed laminate, that is to say a laminate such as is formed by trapping an extruded molten web of plastics material, as it leaves an extruder, between two paper sheets forming part of the laminate.

The term low density as applied to the paper means having a density less than 0.85 g cm'.

The term high impermeability" as applied to the paper means having an impermeability comparable with or higher than that normally used in oil-filled cables and preferably in the higher part of the range, e.g. 10,000 Gurley seconds/mil or higher.

By cellulosic paper" is meant paper consisting substantially entirely of Kraft cellulosic fibres.

One preferred paper is an uncalendered electrical grade paper of intermediate fibre length of a density within the range 0.7 0.85 g cm and an impermeability greater than 10,000 Gurley seconds/mil. Such a paper is Kraft coil winding paper manufactured to the high standard of chemical purity normally associated with capacitor tissue for impregnation with hydrocarbon oils.

The paper layers can be loaded with an active finely divided aluminium oxide or another active metal oxide, hydrated metal oxide, hydroxide, carbonate or basic carbonate that has sorptive powers comparable with that of aluminium oxide, in order to minimise the deterioration in electrical properties due to contamination of the impregnant by residues from the plastics material.

The plastics film is preferably of polyethylene, for instance high density polyethylene, or polypropylene but alternatively polycarbonates, polysulphones, saturated linear polyesters (e.g. polyethylene terephthalate), polyphenylene oxide, poly 4-methylpentene-l polytetrafluoroethylene, ethylene-propylene copolymers, fluorinated ethylenepropylene copolymers, polyfluorostyrene or polyvinylcarbazole can be used.

When the plastics material is of a kind whose physical properties can be improved by treatment with high energy radiation the irradiation can be effected after lamination with the paper.

The conductor screen and/or dielectric screen of cables in accordance with the invention is or are preferably formed from single ply semi-conducting tapes or one or more multiple layers of laminated tape similar to that used to form the whole or part of the dielectric suitably metallised and/or loaded with carbon or other conductive material. Preferably all three layers of the laminate are loaded with conductive material but in some circumstances, for example for the outer layer of a conductor screen or the inner layer of'a dielectric screen, a triple laminate with one paper layer not loaded may be used, that is the paper layers contiguous with the dielectric. The paper not loaded with conductive material is preferably loaded with active finely divided aluminium oxide or other active material of the kind referred to above.

The invention will be further illustrated by a description, by way of example, of a single core 132 kV cable with reference to the accompanying drawing which shows, in

FIG. 1, a cross-section of the cable and, in

FIG. 2 a cut-back end of the cable.

Referring to the drawing the cable comprises an annular conductor 3 of aluminium wires stranded around a tubular helix 2 bounding a central duct 1 of internal .diameter 10 mm for the cable impregnant/refrigerant.

The conductor 3 has an effective cross-sectional area of 600 mm. Surrounding the conductor 3 is a conductor screen 4 built up of two layers of prestressed paper/- polypropylene/paper laminate of thicknesses 25/50/25 um made by the process hereinbefore described and loaded with carbon. Each layer is applied helically with a 65/35 overlap.

The cable dielectric 5 is a laminated dielectric of radial thickness 6.5 mm and is designed for stresses of about 15 Mv/m. The laminated dielectric 5 is built up from a prestressed paper/propylene/paper laminate of thickness 25/50/25 ,u.m made by the process hereinbefore described. Surrounding the laminated dielectric 5 is a dielectric screen 6 built up of two layers of a laminate similar to that employed to build up the conductor screen 4 and applied helically with a similar overlap. The paper used in the laminate of the dielectric 4 and of the conductor and dielectric screens 4 and 6 is an uncalendered electrical grade paper of intermediate fibre length of a density within the range 0.7 to 0.85

gcm and an impermeability greater than 10,000 Gurley seconds/mil.

A corrugated sheath 7 of aluminium encloses the screened insulated conductor. The sheath 7 is surrounded by a layer 8 of thermal insulating material, for instance foamed plastics material, and underlies an oversheath 9 of polyvinyl chloride.

The laminated dielectric 4 is impregnated with liquid nitrogen which is caused to flow along the central duct 1 under a pressure of several atmospheres and which also acts as a cooling medium, in service the cable being maintained at a temperature less than 77K.

An advantage of using composite tape of the triple laminate form (paper/plastics/paper) in a laminated dielectric is that the plastics film contributes high dielectric strength whilst the paper layers, in addition to protecting the plastics film against ionic bombardment should any occur, provide capillary channels to facilitate drying and impregnation of the dielectric and provide the composite tape with the mechanical handling characteristics of paper tapes during lapping or winding to form a laminated dielectric and, in a cable, during subsequent bending of the assembly.

A further advantage arising from the use of composite tape of the triple laminate form in a cable dielectric is that the butt gaps have boundaries substantially wholly of paper, unlike the dielectric formed by interleaving plastics tapes with paper tapes.

Where the plastics film of the composite tape used to make the laminated dielectric of the high voltage cable of the present invention is polyethylene, the laminated dielectric has the additional advantage of increased thermal conductivity over impregnated paper.

What I claim as my invention is:

1. A high voltage cable comprising at least one conductor and a laminated dielectric surrounding the conductor, wherein the dielectric is built up at least in part from tapes comprising an imperforate plastics film having bonded to each of its major surfaces a cellulosic paper tape, impregnated with a liquefied gas which is maintained at a pressure substantially greater than that of the atmosphere.

2. A high voltage cable as claimed in claim 1, wherein the liquefied gas is maintained at a pressure of at least two atmospheres.

3. A high voltage cable as claimed in claim 1, in which an electrically conductive screen overlies the conductor and an electrically conductive screen overlies the laminated dielectric, wherein at least one of the conductor and dielectric screens is formed of at least one layer of single ply semi-conducting tapes.

4. A high voltage cable as claimed in claim 1, wherein the tapes forming at least part of the laminated dielectric are each a prestressed laminate formed by trapping an extruded molten web of plastics material as it leaves an extruded, between two paper sheets forming part of the laminate.

5. A high voltage cable as claimed in claim 1, wherein each cellulosic paper tape of the tapes forming at least part of the laminated dielectric has a density less than 0.85 gcm'.

6. A high voltage cable as claimed in claim 5, wherein each cellulosic paper tape of the tapes forming at least part of the laminated dielectric is an uncalendered electrical grade paper of intermediate fibre length of a density within the range 0.7 0.85 gcm and an impermeability greater than 10,000 Gurley seconds/mil.

7. A high voltage cable as claimed in claim 1, wherein cellulosic paper tapes forming at least part of the laminated dielectric are loaded with a material selected from the group of materials consisting of, aluminium oxide and other active metal oxides, hydrated metal oxides, hydroxides, carbonates or basic carbonates having sorptive powers comparable with that of aluminium oxide.

8. A high voltage cable as claimed in claim 1, wherein the plastics film of the tapes forming at least part of the laminated dielectric is of polyethylene.

9. A high voltage cable as claimed in claim 1, wherein the plastics film of the tapes forming at least part of the laminated dielectric is of polypropylene.

10. A high voltage cable as claimed in claim 1, wherein the plastics film of the tapes forming at least part of the laminated dielectric is irradiated.

11. A high voltage cable as claimed in claim 1, wherein the liquefied gas is selected from the group of liquefied gases consisting of liquid hydrogen, liquid nitrogen, liquid helium, and liquid natural gas.

12. A high voltage cable as claimed in claim 1, wherein the liquefied gas is selected from the group consisting of trifluoromethane, trifluorochloromethane, and difluorochloromethane.

13. A high voltage cable comprising at least one conductor and a laminated dielectric surrounding the conductor, wherein the dielectric is built up at least in part from tapes comprising an imperforate plastics film having bonded to each of its major surfaces a cellulosic paper tape, impregnated with a liquefied gas which is maintained at a pressure substantially greater than that of the atmosphere and the dielectric is surrounded by at least one layer of thermal insulating material which constitutes a component part of the cable.

14. A high voltage cable as claimed in claim 13, in which the cable is enclosed in a sheath, wherein the layer or at least one of the layers of thermal insulating material lies inside the cable sheath.

15. A high voltage cable comprising at least one conductor, an electrically conductive screen overlying the conductor a laminated dielectric surrounding the screened conductor, and a electrically conductive screen overlying the laminate dielectric, wherein the dielectric is built up at least in part from'tapes comprising an imperforate plastics film having bonded to each of its major surfaces a cellulosic paper tape, impregnated with a liquefied gas, which is maintained at a pressure substantially greater than that of the atmosphere, and wherein at least one of the conductor and dielectric screens comprises at least one layer of laminated tape comprising an imperforate plastics film having bonded to each of its major surfaces a cellulosic paper tape rendered electrically conductive.

16. A high voltage cable as claimed in claim 15, wherein all three layers of the or each laminated tape of the screen are loaded with an electrically conductive material.

17. A high voltage cable as claimed in claim 15, wherein at least one of the screen includes a laminated tape immediately adjacent the laminated dielectric of which only the plastics film and the paper tape remote from the laminated dielectric are rendered electrically conductive.

18. A high voltage cable as claimed in claim 17, wherein the paper tape of the or each screen that is contiguous with the laminated dielectric is loaded with amaterial selected from the group of materials consist- 19. A high voltage cable as claimed in claim 15, ing of aluminium oxide and other active metal oxides, wherein at least one layer of thermal insulating material hydrated metal oxides, hydroxides, carbonates or basic surrounds the laminated dielectric and constitutes a carbonates having sorptive powers comparable with component part of the cable. that of aluminium oxide. 5 

2. A high voltage cable as claimed in claim 1, wherein the liquefied gas is maintained at a pressure of at least two atmospheres.
 3. A high voltage cable as claimed in claim 1, in which an electrically conductive screen overlies the conductor and an electrically conductive screen overlies the laminated dielectric, wherein at least one of the conductor and dielectric screens is formed of at least one layer of single ply semi-conducting tapes.
 4. A high voltage cable as claimed in claim 1, wherein the tapes forming at least part of the laminated dielectric are each a prestressed laminate formed by trapping an extruded molten web of plastics material as it leaves an extruded, between two paper sheets forming part of the laminate.
 5. A high voltage cable as claimed in claim 1, wherein each cellulosic paper tape of the tapes forming at least part of the laminated dielectric has a density less than 0.85 gcm
 3. 6. A high voltage cable as claimed in claim 5, wherein each cellulosic paper tape of the tapes forming at least part of the laminated dielectric is an uncalendered electrical grade paper of intermediate fibre length of a density within the range 0.7 -0.85 gcm 3 and an impermeability greater than 10,000 Gurley seconds/mil.
 7. A high voltage cable as claimed in claim 1, wherein cellulosic paper tapes forming at least part of the laminated dielectric are loaded with a material selected from the group of materials consisting of, aluminium oxide and other active metal oxides, hydrated metal oxides, hydroxides, carbonates or basic carbonates having sorptive powers comparable with that of aluminium oxide.
 8. A high voltage cable as claimed in claim 1, wherein the plastics film of the tapes forming at least part of the laminated dielectric is of polyethylene.
 9. A high voltage cable as claimed in claim 1, wherein the plastics film of the tapes forming at least part of the laminated dielectric is of polypropylene.
 10. A high voltage cable as claimed in claim 1, wherein the plastics film of the tapes forming at least part of the laminated dielectric is irradiated.
 11. A high voltage cable as claimed in claim 1, wherein the liquefied gas is selected from the group of liquefied gases consisting of liquid hydrogen, liquid nitrogen, liquid helium, and liquid natural gas.
 12. A high voltage cable as claimed in claim 1, wherein the liquefied gas is selected from the group consisting of trifluoromethane, trifluorochloromethane, and difluorochloromethane.
 13. A high voltage cable comprising at least one conductor and a laminated dielectric surrounding the conductor, wherein the dielectric is built up at least in part from tapes comprising an imperforate plastics film having bonded to each of its major surfaces a cellulosic paper tape, impregnated with a liquefied gas which is maintained at a pressure substantially greater than that of the atmosphere and the dielectric is surrounded by at least one layer of thermal insulating material which constitutes a component part of the cable.
 14. A high voltage cable as claimed in claim 13, in which the cable is enclosed in a sheath, wherein the layer or at least one of the layers of thermal insulating material lies inside the cable sheath.
 15. A high voltage cable comprising at least one conductor, an electrically conductive screen overlying the conductor , a laminated dielectric surrounding the screened conductor, and a electrically conductive screen overlying the laminate dielectric, wherein the dielectric is built up at least in part from tapes comprising an imperforate plastics film having bonded to each of its major surfaces a cellulosic paper tape, impregnated with a liquefied gas, which is maintained at a pressure substantially greater than that of the atmosphere, and wherein at least one of the conductor and dielectric screens comprises at least one layer of laminated tape comprising an imperforate plastics film having bonded to each of its major surfaces a cellulosic paper tape rendered electrically conductive.
 16. A high voltage cable as claimed in claim 15, wherein all three layers of the or each laminated tape of the screen are loaded with an electrically conductive material.
 17. A high voltage cable as claimed in claim 15, wherein at least one of the screen includes a laminated tape immediately adjacent the laminated dielectric of which only the plastics film and the paper tape remote from the laminated dielectric are rendered electrically conductive.
 18. A high voltage cable as claimed in claim 17, wherein the paper tape of the or each screen that is contiguous with the laminated dielectric is loaded with a material selected from the group of materials consisting of aluminium oxide and other active metal oxides, hydrated metal oxides, hydroxides, carbonates or basic carbonates having sorptive powers comparable with that of aluminium oxide.
 19. A high voltage cable as claimed in claim 15, wherein at least one layer of thermal insulating material surrounds the laminated dielectric and constitutes a component part of the cable. 